1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting and receiving packet data using Hybrid Automatic Repeat Request (HARQ). In particular, the present invention relates to an apparatus and method for transmitting and receiving packet data using HARQ in a mobile communication system.
2. Description of the Related Art
Presently, HARQ is one of the most important technologies used for increasing data transmission reliability and data throughput in packet-based mobile communication systems. Specifically, HARQ refers to a combined technology of Automatic Repeat Request (ARQ) and Forward Error Correction (FEC). Automatic Repeat Request (ARQ) is a technology that is widely used in wired/wireless data communication systems, and a brief description thereof will now be made herein below.
A transmitter of a system using ARQ assigns a sequence number to a transmission data packet according to a predetermined rule. A data receiver of the system using ARQ can detect a missing packet (or a reception-failed packet) among the received packets using the sequence number. Accordingly, a packet corresponding to a missing sequence number among the transmitted sequence numbers is the missing packet that the data receiver has failed to receive. In this case, the data receiver sends a request for retransmission of the reception-failed packet with the missing sequence number to the transmitter. In this way, the system using ARQ achieves reliable data transmission.
In the foregoing description, FEC refers to a technology for adding redundant bits to transmission data according to a predetermined rule before transmission, like convolutional coding or turbo coding, to overcome an error occurring in a noise or fading environment in a data transmission/reception process, thereby demodulating the originally transmitted data.
Therefore, the system using HARQ denotes a system using the combined technology of ARQ and FEC. A description will now be made of exemplary transmission/reception of packet data in a system using HARQ.
The data receiver of a system using HARQ performs decoding on a received packet through an inverse FEC process, and determines whether there is any error in the decoded data through a Cyclic Redundancy Check (CRC). If there is no error as a result of the CRC check, the data receiver responds with an Acknowledgement (ACK) to the transmitter over a response channel so that the transmitter can then transmit the next data packet. However, if there is an error in the received data as a result of the CRC check, the receiver responds with a Non-Acknowledgement (NACK) to the transmitter over the response channel so that the transmitter retransmits the previously transmitted packet.
In the foregoing process, if the received packet is a retransmitted packet, the receiver combines the received packet with the previously transmitted packet thereby obtaining an energy gain. As a result, the HARQ obtains highly improved performance, compared with the ARQ which does not support the combining process.
The HARQ technologies are classified according to a number of classification criterion, such as time interval between initial transmission and retransmission. A description will now be made of Synchronous HARQ (SHARQ) and Asynchronous HARQ (AHARQ) which are classified according to time interval between initial transmission and retransmission.
FIG. 1 is a diagram illustrating exemplary timing that occurs during SHARQ packet transmission/reception. With reference to FIG. 1, a description will now be made of exemplary SHARQ packet transmission/reception.
In FIG. 1, a horizontal axis indicates a time axis, of which an upper side of the horizontal axis shows a channel transmitted from a transmitter, and a lower side of the horizontal axis shows a channel transmitted from a receiver. In addition, an arrow denotes a transmission process from the transmitter to the receiver, or from the receiver to the transmitter.
An exemplary transmitter of a system using SHARQ transmits a control channel and a data channel at an initial transmission in step 101. The data channel is a channel used for transmitting the packet generated using transmission data, and the control channel is a channel used for transmitting the information necessary for demodulation and decoding of the packet transmitted through the data channel. The control information generally used in the system using HARQ is shown by way of example in Table 1 below.
TABLE 1Control information included incontrol channelNumber of allocated bitsMS ID Information10Data Information Size6MCS Information5Used Resource Information5(or Resource Allocation Information)ARQ ID3Sub-Packet ID3
A detailed description of the exemplary information shown in Table 1 will now be made below.
The MS ID denotes an identifier (ID) allocated for mobile station (MS) identification, predefined between a base station (BS) and an MS. The MS ID is included when the base station transmits data to the mobile station. However, the MS ID Information can be omitted from data control information, when the mobile station transmits data to the base station, and when the base station previously orders the mobile station to transmit data through a specific resource at a specific time.
The Data Information Size denotes the number of data bits transmitted in a given transmission interval. The Modulation and Coding Scheme (MCS) information denotes modulation and coding schemes used for data channel transmission. For example, the MCS Information indicates which of the various modulation schemes such as Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK), 16-ary Quadrature Amplitude Modulation (16 QAM), and 64-ary QAM (64QAM) is used, and which of the various coding schemes such as convolutional coding and turbo coding is used. The Used Resource Information denotes the amount of wireless resources used for data channel transmission. For example, in Code Division Multiple Access (CDMA), the Used Resource Information indicates the number of Walsh codes used for data channel transmission, and in Orthogonal Frequency Division Multiple Access (OFDMA), the Used Resource Information indicates the number of sub-carriers used for data transmission and their location information. The ARQ ID denotes an ID for each ARQ process when several ARQ processes are simultaneously supported, and the Sub-Packet ID denotes an ID used for identifying each retransmission among several retransmissions of one packet data.
FIG. 1 illustrates exemplary initial transmission steps 101 and 107, and first and second retransmission steps 103 and 105, respectively. It is shown in FIG. 1 that the control channel and the packet data channel are transmitted together only at the initial transmissions of steps 101 and 107. That is, it can be assumed for example that the control channel is not transmitted at retransmission. However, in some cases, the control channel can be transmitted together with the data channel even at retransmission.
Generally, in the system using HARQ, during data transmission, control information necessary for demodulation of the data channel is simultaneously transmitted over a control channel. However, the types of control information and the number of bits thereof, as shown in Table 1, are subject to change.
As shown in FIG. 1, if a transmitter (not shown) transmits a control channel and a data channel at initial transmission of step 101, a receiver (not shown) receives the control channel and the data channel. The receiver preferentially demodulates the control channel transmitted at the initial transmission of step 101. By demodulating the control channel, the receiver acquires control information necessary for demodulation of the data channel, and performs demodulation and decoding on a packet received through the data channel using the acquired control information. After the demodulation and decoding of the packet received through the data channel, the receiver determines whether the received data is successfully demodulated and decoded, by performing a CRC check on the decoded data.
In step 102, it is shown that demodulation and decoding at the receiver on the packet transmitted at the initial transmission of step 101 has failed. That is, the receiver transmits a NACK through a response channel in step 102. In response thereto, the transmitter performs a first retransmission in step 103. It can be assumed for example that the transmitter transmits only the data channel during retransmission. The receiver then demodulates and decodes the packet transmitted through the data channel, and transmits an ACK or a NACK according to a CRC result thereon in step 104.
For exemplary purposes, it is also shown in FIG. 1 that demodulation and decoding at the receiver on the first retransmitted data fails. Therefore, the receiver again generates a NACK and transmits the NACK to the transmitter in step 104. The transmitter then performs a second retransmission in step 105. As above, the transmitter transmits only the data channel, because the corresponding transmission is a retransmission. Through this process, if the receiver succeeds in demodulation and decoding on the packet received through the data channel, it transmits an ACK through a response channel in step 106. The transmitter can then perform initial transmission in step 107 after it receives the ACK in step 106.
The transmitter transmits a control channel and a data channel together at the initial transmissions in steps 101 and 107. The transmitter can perform the initial transmission of step 107 immediately upon receipt of the ACK in step 106, or perform the initial transmission after a lapse of a certain period of time. The next initial transmission time is determined depending on the scheduling result. In HARQ, because the number of retransmissions is determined according to the system, transmissions are limited to the number of retransmissions.
The information transmitted through the data channel in steps 101, 103 and 105 is the same information. However, it should be noted herein that even though the same information is transmitted through the data channel, different redundancies can be used. Each packet comprised of the same information represented by data transmissions 101, 103 and 105 for transmitting the same information is called a sub-packet.
In SHARQ, it should be noted that a time interval between steps 101, 103 and 105 is constant. That is, a time interval between initial transmission and retransmission or a time interval between retransmissions remains constant. In other words, in the SHARQ, only the initial transmission time is determined by a scheduler, and a retransmission packet for the initially transmitted packet is automatically transmitted after a lapse of a predetermined period of time from the initial transmission time. The expression “synchronous” in the term SHARQ indicates that the time interval is constant.
Therefore, the term “SHARQ” cannot be restrictively considered as a scheme that transmits control information only at the initial transmission and transmits no control information at retransmission as described in FIG. 1. However, unless stated otherwise, the term SHARQ used herein is considered to denote a scheme that transmits control information only at the initial transmission and transmits no control information at retransmission as described in FIG. 1, but is not limited thereto.
The SHARQ scheme transmits no control information during retransmission because for the retransmission sub-packets, their control information is the same information as that used at the initial transmission or is information that can be estimated. Therefore, in the case of the first retransmission, i.e. in the case of step 103, for the retransmitted sub-packet, the receiver can combine it with the sub-packet transmitted through the data channel at the initial transmission in step 101 according to a predetermined rule. The receiver attempts demodulation of the data channel using the combined result. Thereafter, the receiver demodulates and decodes the combined packet, and determines whether the data transmission is successful using the CRC result on the demodulated and decoded packet. That is, the receiver determines whether it has succeeded in decoding the combined sub-packet using the CRC result. The same is applied even to the sub-packet retransmitted afterward. That is, the receiver combines the sub-packet retransmitted in step 105 with the previously transmitted sub-packets, performs demodulation and decoding on the combined sub-packet, and determines whether the data transmission is successful using the CRC result.
FIG. 2 is a diagram illustrating exemplary timing during AHARQ packet transmission/reception. With reference to FIG. 2, a description will now be made of AHARQ packet transmission/reception.
In FIG. 2, a horizontal axis indicates a time axis, of which an upper side of the horizontal axis shows a channel transmitted from a transmitter, and a lower side of the horizontal axis shows a channel transmitted from a receiver. In addition, an arrow denotes a transmission process from the transmitter to the receiver, or from the receiver to the transmitter.
A transmitter (not shown) performs an initial transmission in step 201. Here, it can be assumed for example that the initial transmission is comprised of a data channel and a control channel. A receiver (not shown) receives the data channel transmitted in step 201, and demodulates the control channel and acquires control information necessary for demodulation of the data channel. In addition, the receiver attempts demodulation and decoding on the data channel using the control information. In this process, the receiver determines whether it has succeeded in demodulating and decoding the packet using the CRC result on the demodulated and decoded packet. If it is determined from the CRC result that the packet transmitted through the data channel is not successfully demodulated and decoded, the receiver transmits a NACK to the transmitter through a response channel.
For exemplary purposes, it is shown in FIG. 2 that the demodulation and decoding at the receiver has failed. Therefore, the receiver transmits a NACK to the transmitter through the response channel in step 202. Upon receipt of the NACK, the transmitter performs retransmission on the initially transmitted packet in step 203. It should be noted in AHARQ that a time interval between the initial transmission in step 201 and the retransmission in step 203 is not constant as shown in FIG. 2. That is, the transmitter can transmit a different retransmission packet each occasion. In other words, compared with the HARQ scheme shown in FIG. 1 where only the initial transmission time is scheduled and the retransmission time is automatically determined from the initial transmission time, the AHARQ scheme shown in FIG. 2 determines not only the initial transmission time, but also all retransmission times using a scheduler.
Therefore, once a first retransmission time is determined by the scheduler, the transmitter transmits the control channel and the data channel together. in step 203. The receiver can then perform demodulation and decoding on the packet transmitted through the data channel using the received control channel, and detect the demodulation and decoding result using a CRC.
For exemplary purposes, it is shown in FIG. 2 that demodulation and decoding on the first retransmitted packet at the receiver has failed. Therefore, the receiver transmits a NACK through a response channel in step 204. The transmitter then determines a second retransmission time using the scheduler, and performs a second retransmission in step 205. It is shown in FIG. 2 that a time interval between the first retransmission and the second retransmission is constant as shown in FIG. 1. That is, in terms of the retransmission time, the AHARQ scheme can be equal to the SHARQ scheme.
The receiver receives the sub-packet transmitted in step 205, and demodulates and decodes the received sub-packet. The receiver determines in step 206 whether it has succeeded in the demodulation and decoding through a CRC of the demodulated and decoded packet. It can be assumed for example in step 206 of FIG. 2 that the CRC result indicates success in the demodulation and decoding. Therefore, the receiver transmits an ACK through a response channel in step 206. The retransmitted sub-packets, as described in FIG. 1, are combined by the receiver and then demodulated and decoded.
It can be noted from FIG. 2 that the transmitter performs retransmission in step 203 regardless of the initial transmission time of step 201. It would be obvious to those skilled in the art that the transmission time of step 203 can be determined to be equal to the time shown in step 103 of FIG. 1. It is also well known that the retransmission is performed after a NACK is received from the receiver.
Since the retransmission time is also scheduled as described above, the AHARQ scheme is characterized in that the control channel should be transmitted together during every sub-packet transmission. This means that the control channel including ID information for the transmitted packet is always transmitted together, because as the retransmission time is not determined, the receiver cannot be aware of the time that the retransmission packet is transmitted.
This also means that the AHARQ scheme is disadvantageously greater than the SHARQ in terms of overhead. However, the AHARQ scheme is advantageously greater than the SHARQ scheme in terms of scheduling freedom. The greater scheduling freedom indicates that the scheduler freely determines a retransmission time so as to first transmit high-priority packets and transmit the retransmission packet later.
A description will now be made of exemplary advantages and disadvantages of the SHARQ scheme and the AHARQ scheme of FIGS. 1 and 2.
The SHARQ scheme has an advantage of low overhead because it transmits the control channel only at the initial transmission. However, the AHARQ scheme has an advantage of high scheduling freedom because the scheduler can determine the retransmission time.
In addition to the above schemes, there are various service types available in the data transmission system. For example, there is real-time service that is susceptible to data transmission time delay, and a best-effort service that is unsusceptible to transmission time delay. In addition, the real-time service is generally characterized in that small packets are frequently generated. Therefore, the SHARQ scheme is advantageous to the real-time service where the small packets are generated frequently, and the AHARQ scheme is advantageous to the service less susceptible to time delay. However, the common mobile communication system uses only one of the SHARQ scheme and the AHARQ scheme. Therefore, the SHARQ and AHARQ schemes, when only one of them is used, are inefficient in an environment where several types of services are provided to several users in a mobile communication system. In addition, because the types of data services provided to users are being subdivided, it is not possible to actively cope with data transmission with only one HARQ scheme.
Accordingly, a need exists for an improved system and method for using HARQ in a mobile communication system where several types of services are provided to several users.